Kernel: Fix allocating identity-mapped APIC memory on x86_64

We were not allocating enough memory due to using u32 instead of
FlatPtr for each AP's stack pointer.

Author: tomuta

Kernel/Interrupts/APIC.cpp

@@ -345,8 +345,9 @@ UNMAP_AFTER_INIT void APIC::setup_ap_boot_environment()
     // * aps_to_enable u32 values for ap_cpu_init_stacks
     // * aps_to_enable u32 values for ap_cpu_init_processor_info_array
     constexpr u64 apic_startup_region_base = 0x8000;
-    VERIFY(apic_startup_region_base + apic_ap_start_size < USER_RANGE_BASE);
-    auto apic_startup_region = create_identity_mapped_region(PhysicalAddress(apic_startup_region_base), Memory::page_round_up(apic_ap_start_size + (2 * aps_to_enable * sizeof(u32))).release_value_but_fixme_should_propagate_errors());
+    auto apic_startup_region_size = Memory::page_round_up(apic_ap_start_size + (2 * aps_to_enable * sizeof(FlatPtr))).release_value_but_fixme_should_propagate_errors();
+    VERIFY(apic_startup_region_size < USER_RANGE_BASE);
+    auto apic_startup_region = create_identity_mapped_region(PhysicalAddress(apic_startup_region_base), apic_startup_region_size);
     u8* apic_startup_region_ptr = apic_startup_region->vaddr().as_ptr();
     memcpy(apic_startup_region_ptr, reinterpret_cast<const void*>(apic_ap_start), apic_ap_start_size);